This invention relates to lead-free solder alloy powder usage in pastes for soldering components to printed circuit boards (PCB""s).
When lead-free solders are used in the soldering of components to printed circuit boards, problems stem from the increase in reflow (soldering) temperature required by the main lead-free alternatives relative to the eutectic Sn(PbAg) solders currently used. The principal lead-free solders are based on the tin-copper, tin-silver and tin-silver-copper eutectics, melting at 227xc2x0 C., 221xc2x0 C. and 217xc2x0 C., respectively. These alloys require relatively high reflow temperatures in the range 230-240xc2x0 C. to obtain adequate soldering. Reflow temperatures in this range may damage temperature sensitive components. Thus, lead free solders have not yet found widespread adoption in electronics assembly despite their obvious advantages.
In order to lower lead-free alloy melting points below 217xc2x0 C., and accordingly, lower reflow temperatures, it is possible to alloy the above mentioned lead-free solders with limited amounts of bismuth, generally up to about 10%, typically 2-5% by weight. There is indeed an additional advantage in using bismuth in solid solution in tin in lead-free alloys in that the bismuth has been found to strengthen the lead-free alloys. The addition of bismuth has, however, the disadvantage of introducing a melting range. Thermal strains developing in an assembly on cooling, in conjunction with solder joints with a freezing range can, in unfavorable circumstances, lead to hot cracking. This phenomenon of hot cracking has been observed particularly in plated-through-hole applications but not in surface mount reflow soldering, so bismuth-containing alloys are viable in many solder paste applications.
A defect commonly found on reflow soldered printed circuit boards is that when chip components solder faster at one end than another and as a consequence are pulled by surface tension, they stand vertically on one pad, creating an electrical discontinuity. It is for this reason that this behavior is known as tombstoning. In current practice with lead-containing solders, this is addressed by using a solder alloy with a melting range, but, where lead-free solders are concerned, no solution has been proposed.
Snxe2x80x94Agxe2x80x94Bi and Snxe2x80x94Agxe2x80x94Cuxe2x80x94Bi alloy compositions are well documented as lead-free solder alloys. The concept of combining high and low melting point alloy powders to form a mixture which melts at a low temperature but forms a high melting point joint is also well documented. However, the use of such alloys with a melting range to inhibit tombstoning is not known, and has not been documented.
We have found that solder pastes made with mixtures of Sn63Pb37 powder and Sn62Pb36Ag2 powder, melting at 183xc2x0 C. and 179xc2x0 C. respectively surprisingly give a significantly reduced tendency to tombstoning as compared to pastes made up with prealloyed powders, when used to solder chip components on printed circuit boards.
On this basis we have concluded that use of a mixture of a SnBi alloy powder, such as a Sn43Bi57 powder, and a SnAgCu alloy powder, such as Sn96.5Ag3.8Cu0.7 powder, such specific alloys melting at 138xc2x0 C. and 217xc2x0 C. respectively, in a (lead-free) paste, will analogously also have an anti-tombstoning effect as well as enable the paste to start to reflow at a lower temperature than if prealloyed powder were used.